The invention relates generally to photocapacitors that employ variation in capacitance. In particular, capacitance responds to variation in light intensity primarily, but in the alternate also responds to variation in light frequency.
Photocapacitance has been used by the scientific community for many years to investigate important aspects of semiconductor materials. By scanning the frequency of the light, various deep-level traps can be identified and characterized. For purposes of this disclosure, the term “trap” inclusively means either a trap or a recombination center, the distinction of which being that a trap generally interacts with only one species of (either) electrons or (else) holes, and a recombination center interacts with both electron and holes. In the literature, the terms are often used interchangeably.
For example, when a sudden change in capacitance is observed, the correlation to the photon energy of the light (determined by Ehν=hν where h is Planck's constant and ν is the frequency) reveals the activation energy of a state within the band-gap of the material. Further, by identifying the sign of change in the capacitance, the type of trap can be determined, i.e., donor or acceptor-like. In particular, a donor type constitutes a neutral trap when filled with an electron and positively charged when empty; and an acceptor-like type represents a neutral trap when filled with a hole and negatively charged when empty.
Photocapacitance can be used to determine other information as well. There are approximately thirty variations of the photocapacitance method to determine material properties. Summary information of techniques can be found in references: P. Blood and J. W. Orton, “The electrical characterization of semiconductors,” Rep. Prog. Phys., 41, 2, pp. 157-258 (1978), and C. T. Sah, L. Forbes, L. L. Rosier and A. F. Tasch, Jr., “Thermal and Optical Emission and Capture Rates and Cross Sections of Electrons and Holes at Imperfection Centers in Semiconductors from Photo and Dark Junction Current and Capacitance Experiments,” Solid-State Electronics, 13, 6 (1970), pp. 759-88.
As mentioned, a common use of photocapacitance in the scientific community is to determine inter-band-gap state information in a semiconductor. FIGS. 1A and 1B show inter-band-gap state information of a semiconductor material, and the corresponding photocapacitance data, obtained from Blood et al. FIG. 1A shows a representational view 100 of an electron band-gap 110 between an upper conduction band edge 120 and a lower valance band edge 130.
A photon 140 having energy Ehν>0.465 eV strikes a trap within the bandgap 110 to ionize position N1+ at 0.465 eV transferring to the resultant electron 145 sufficient energy for quantum transition to the conduction band edge 120. FIG. 1B shows a graphical view 150 of normalized capacity as a function of photon energy Ehν, with the latter as the abscissa 160 and the former as the ordinate 170.
A series of stepped electron capacitance values 180 demonstrates a series of normalized capacity values from 0.4 eV to about 1.6 eV. Several states are identified at photon energies of 0.465 eV, 0.73 eV, 0.78 eV etc. The state at 0.73 eV shows a positive change in capacitance signifying a donor-like state. By contrast, the nearby state at 0.78 eV shows a negative change in capacitance signifying an acceptor-like state. In this manner, a photocapacitor can respond to the photon energy (frequency) as provided in various exemplary embodiments. In addition, other exemplary embodiments provide for controlling capacitance by changing the light intensity.